Removing hydrogen sulfide from a gaseous mixture using iron hydroxide bonded to calcined diatomite

ABSTRACT

A process for removing hydrogen sulfide from a gas stream wherein the filtering media consists essentially of calcined diatomite and between 5% and 30% by weight of ferric ions bonded to the calcined diatomite. This process is particularly efficient due to the fact that the removal of hydrogen sulfide from the gas stream is effected with a single pass adsorption performance of up to 45 mg of H 2 S per gram of filtering media. Other advantages include the fact that the process has the ability to remove H 2 S form a gaseous mixture, from a concentration of 30,000 ppm down to non-detectable levels in a single pass. Repeated in-situ regeneration of the filtering media has been proven to be a simple matter of blowing ambient air through the filtering media.

[0001] This is a continuation-in-part of U.S. patent application Ser.No. 09/407,708 filed on Sep. 28, 1999, which was a continuation-in-partof U.S. patent application Ser. No. 08/909,819, filed on Aug. 12, 1997.

FIELD OF THE INVENTION

[0002] The present invention relates to the removal of hydrogen sulfide(H₂S) from various gases, and more particularly it relates to theremoval of hydrogen sulfide from a gaseous mixture using a filteringmedia containing iron hydroxide intimately bonded to calcined diatomite.

BACKGROUND OF THE INVENTION

[0003] Hydrogen sulfide can be present in various air streams and isoften found in gas streams associated with petroleum storage andtransfer facilities, anaerobic digesters, sewage treatment plants andpulp and paper mills. In many cases, the hydrogen sulfide has to beremoved because of its toxicity, corrosive properties, and unpleasantodour.

[0004] Several methods are known and have been used in the past forremoving hydrogen sulfide from a gas stream. Perhaps the most popularmethod is one which consists in passing the gaseous mixture through aniron sponge bed. The iron sponge bed is a type of filter which comprisesessentially iron salts adsorbed on a wood chip support media. Also,there are known processes in which a sulfurous gas is passed through abed of iron oxide particles. These processes and various others aredescribed in U.S. Patents which can be found in particular in the U.S.Classification 423/231 entitled: Removing Hydrogen Sulfide from aGaseous Mixture Utilizing Iron Oxide or Hydroxide.

[0005] Problems associated with the prior art processes are numerous andinclude the facts that some media are self igniting when exposed to airand therefore are not renewable. Because of their weak sulfur retention,some of these non-renewable media must be treated as hazardous waste.Other known filtering media have a relatively low H₂S adsorptioncapacity or a low H₂S adsorption performance in a single pass process.Another drawback of some commercial H₂S filtering systems is that thefiltering media must be disposed of after a single use.

[0006] As such, it will be appreciated that there continues to be a needfor a filtering process in which the filtering media is capable ofremoving hydrogen sulfide from a gaseous mixture with a high single passperformance. Further, it is believed that there continues to be a needfor a filtering process wherein the filtering media is easily renewableand does not generate any hazardous waste when disposed of aftermultiple reuses.

SUMMARY OF THE INVENTION

[0007] The present invention provides for an effective process forremoving hydrogen sulfide from a gas stream. Essentially, the processaccording to the present invention uses a filtering media which has alarge single-pass adsorption performance, which is renewable severaltimes and which has a considerable lifetime adsorption capacity.

[0008] In a first aspect of the present invention, there is provided aprocess for removing hydrogen sulfide from a gas stream wherein the gasstream is passed through a filtering media consisting essentially ofcalcined diatomite and between 5% and 30% by weight of ferric ionsbonded to the calcined diatomite.

[0009] This process is particularly efficient due to the fact that theremoval of hydrogen sulfide from the gas stream is effected with asingle pass adsorption performance of up to 45 mg of H₂S per gram offiltering media. Other advantages include the fact that the process hasthe ability to remove H₂S from a gaseous mixture, from a concentrationof 30,000 ppm down to non-detectable levels of less than 0.2 ppm in asingle pass. Further, the lifetime adsorption capacity of the filteringmedia is about one half or more of the weight of the filtering media.

[0010] In another aspect of the present invention, there is provided aprocess for removing hydrogen sulfide from a gas stream, comprising thesteps of passing the gas stream through a filtering media consistingessentially of calcined diatomite, and between 5% and 30% by weight offerric ions bonded by chemisorption bonds to the calcined diatomite. Theprocess further includes the steps of renewing the filtering mediaseveral times when the filtering media is saturated with sulfides byblowing ambient air through the filtering media. The regeneration of thefiltering media can be accomplished while maintaining an averagehydrogen sulfide adsorption performance thereof of about 32 mg ofhydrogen sulfide per gram of filtering media per cycle. Because thefiltering media is non-flammable, there is no risk of combustion due tothe heat generated during the regeneration process.

[0011] In yet another aspect of the present invention, there is provideda process for removing hydrogen sulfide from a gas stream, comprisingthe step of passing the gas stream through a filtering media consistingessentially of calcined diatomite having particles ranging in sizesbetween about 30 mesh and about 60 mesh, and between 5% and 30% byweight of ferric ions bonded by chemisorption bonds to the calcineddiatomite. The process is particularly advantageous for filtering moistgases, due to the fact that the filtering media remains porous when wet.The efficiency of the process increases with the adsorption of a certainamount of moisture in the filtering media.

[0012] Still another feature of the process according to the presentinvention is that it is susceptible of a low cost of material,installation and operation, and accordingly is then susceptible of lowprice of sale to the industry, thereby making such H₂S filtering processeconomically available to the public.

[0013] Other advantages and novel features of the invention will becomeapparent from the following detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] While this invention is susceptible of embodiments in manyvarious forms, there will be described in details herein a specificembodiment, with the understanding that the present disclosure is to beconsidered as an example of the principles of the invention and is notintended to limit the invention to the embodiment described.

[0015] The filtering media used in the process according to preferredembodiment was the subject of an earlier patent application, Ser. No.09/407,708, filed in Sep. 28, 1999, in which it is described as afiltering media for removing arsenic from ground water. The filteringmedia is known under the trademark MEDIA G2® and is available from ADIInternational Inc., a corporation having its principal place of businessin Fredericton, New Brunswick, Canada. Although the initial purpose ofthe filtering media was for use as a water filter, it was discoveredthat the same filtering media has advantageous properties in removinghydrogen sulfide from a gas stream.

[0016] The filtering media contains iron hydroxide Fe(OH)₃ intimatelybonded to calcined diatomite. The ferric ions content in the filteringmedia is between about 5% and about 30% by weight. The preferredcalcined diatomite material has particles ranging in sizes from about 30mesh to about 60 mesh. This size and type of diatomite particles hasbeen found to be advantageous for use in filtering arsenic from water aswell as for use in gas filtering columns, particularly for allowingintimate contact between the gas and the ferric ions bonded to thediatomite particles. The calcined diatomite particles do not offersubstantial resistance to the flow of a gas passing through it and donot expand in contact with a moist gas.

[0017] The calcined type of diatomite is believed to be an importantelement also contributing to the performance of filtering media inremoving hydrogen sulfide from sulfurous gases. Calcined diatomiteparticles have multiform shapes and a greater porosity than ordinarydiatomite particles. The heat treatment applied to the diatomiteparticles during the calcination process, increases the porosity of theparticles by breaking their surfaces and forming pores, cracks,crevices, cavities, hollows and protrusions. These pores, cracks,crevices, cavities, hollows and protrusions offer additional surfaces oneach particle to adsorb and to retain ferric ions.

[0018] During the manufacturing of the filtering media, the calcineddiatomite is impregnated with ferric ions in a liquid form, such as aferric chloride solution. The solution entrains the ferric ions over theentire surface of the calcined diatomite particles and deep inside thepores, cracks, crevices and cavities of the particles. Then, the ferricchloride is converted into iron hydroxide in-situ within the diatomiteparticles, using sodium hydroxide for example, to better bond the ferricions over and into the entire structure of each diatomite particle. Thediatomite material has negative charges and attracts the positivelycharged ferric ions, thereby contributing to the formation of strongionic impregnation bonds between the diatomite material and the ferricions.

[0019] During the manufacturing process, the sodium hydroxide is addedslowly to bring the pH of the slurry to a final value of at least about9, ensuring a complete and unhasty conversion of the ferric chloride toiron hydroxide. This manufacturing process is advantageous for yieldinga ferric ion content, in the form of iron hydroxide, of between about 5%and about 30% by weight of the media. During this manufacturing process,it is believed that irregular clusters of ferric ions are formed andbecome entrapped or otherwise interlocked inside the pores, cracks,crevices and cavities of the calcined diatomite particles, and thereforebecome strongly bonded to the calcined diatomite particles. The bonddescribed above is believed to be a chemisorption bond produced by animpregnation-oxidation process which is characterized by itsirreversible chemical forces. It is also believed that thesechemisorption bonds between the ferric ions and the calcined diatomiteparticles contribute greatly to the abilities of the filtering media toretain its ferric ions such that it is renewable several times afterbeing saturated with sulfides. Also it was found that when the filteringmedia is saturated with water or subjected to a flow of water throughit, the iron component is not released from the filtering media. Thisfinding supports the fact that iron hydroxide is intimately bonded tothe calcined diatomite particles.

[0020] Other advantages of using the filtering media for removing H₂Sfrom a gas stream include the fact that it has a pH of about 10-11,which favourably affect the reaction of H₂S with the ferric ions in thefiltering media.

[0021] Testing of the filtering media was carried out to determine itsoptimum performance in removing hydrogen sulfide from a sulphurous gas.In a first series of tests to determine adsorption capacities,comparative results were obtained from similar tests carried on avariant of the filtering media, hereinafter referred to as the variantmedia, wherein the calcined diatomite was replaced by vermiculite. Thefiltering media and the variant media are generally or jointly referredto as the media sample or both media samples.

[0022] Both media samples were subjected to testing in dry and moistconditions. Dry columns were filled with media samples that were notrinsed or pre-moistened. All the fines were present in the mediasamples. Moist columns were prepared in three different ways: 1) washingthe media samples with water until all the fines were removed; 2)soaking the media samples in water overnight and placing them in thecolumns with minimal removals of the fines, and 3) placing the mediasamples into the columns and then pouring water down through them. Inall three cases, the moist columns were allowed to drain out all excesswater for at least one day before testing began.

[0023] All columns were fed a gas stream from an anaerobic digester,containing approximately 30,000 ppm of H₂S. The outlet H₂S concentrationwas measured several times per day, using gas testing tubes known underthe trade name DRAEGERT®, and having a minimum readable value of 0.2ppm. Saturation of the media samples was determined when H₂Sconcentration in the treated gas exceeded 500 ppm.

[0024] The results of the tests were as follows. The adsorptionperformance of the column containing the washed filtering mediaaccording to the preferred embodiment was approximately 30 mg of H₂S pergram of filtering media before it was considered saturated. Thefiltering media which had been soaked but not rinsed of fines had anadsorption performance of 45 mg of H₂S per gram of filtering media. Thedry filtering media adsorbed about 40 mg of H₂S per gram of filteringmedia. The variant media was able to adsorb almost 70 mg of H₂S per gramof variant media.

[0025] It is believed that during the filtration process, the mediasamples adsorb H₂S and form ferric sulfide by the oxidation of H₂S andthe dissociation of the iron hydroxide species present in the mediasamples.

[0026] Regeneration of the filtering media was accomplished by theoxidation of the FeS produced during H₂S removal. FeS was oxidized bysimply blowing ambient air through the column to form different speciesof iron hydroxide, elemental sulphur and water. The regeneration processreconverts the iron hydroxide to its original bond to the diatomitematerial such that the filtering media is usable again to remove H₂Sfrom a sulfurous gas stream.

[0027] Ferric sulfide is grey to brownish black in colour andagglomerates into lumps, rods or granular powder during the filtrationprocess. As a sulfurous gas is passed upward through the filteringmedia, the filtering media gradually turns black, beginning at thebottom of the column, and indicates that H₂S removal is taking place.During the process, the formation of lumps and fine clay-like graypowder can also be noticed.

[0028] Some of the test columns were transparent. During regeneration ofthe filtering media, a colour change was also noted. The filtering mediawas seen to change from completely black to almost its original orangecolour. After regeneration, the filtering media may have a lighter shadeof orange, possibly due to the elemental sulphur, being yellow, producedin the filtering media. In some cases, black specs may remain in thefiltering media. These black specs indicate that total regeneration hasnot been attained.

[0029] Attempts to regenerate the variant media were unsuccessful.Blowing air through the column did not return the variant media to itsoriginal colour. Also, it was found that as this variant media becomeswet, it expands to such a degree that the filter column becomescompletely plugged, and the flow of gas there through becomes almostimpossible.

[0030] The testing of the filtering media has indicated that theadsorption performance of the dry filtering media was similar to that ofthe pre-moistened filtering media, being 40 mg of H₂S and 45 mg of H₂Sper gram of filtering media respectively. The performance of the dryfiltering media has been shown to increase to a same level as for themoist filtering media, after it had adsorbed moisture from the gasstream passing through it. It is believed that the slight difference ininitial performance is compensated for by the advantages in eliminatingthe need for pre-moistening the filtering media.

[0031] The testing of the filtering media and the variant media alsoindicated that although the variant media had the ability to absorb moreH₂S in a first run, it is not renewable and therefore, the adsorptioncapacity of the filtering media according to the preferred embodimentexceeds that of the variant media in only two cycles. One cycle isreferred to as a saturation of the filtering media with sulfides and theregeneration of the filtering media.

[0032] A second part of the testing program was focussed on themechanical characteristics of the filtering process, and moreparticularly it was focussed on finding an optimum empty bed contacttime (EBCT). The EBCT is defined as the residence time of the gas insidethe filtering column.

[0033] Three different columns of one half inch in diameter and fivefeet tall each were set up to test the effect of EBCT. These threecolumns were set up to run at 40, 130 and 200 ml/min, yielding EBCT of300, 90 and 60 seconds, respectively. Again, all columns were fed a gasstream from an anaerobic digester, containing approximately 30,000 ppmof H₂S. The filtering media in all three columns were pre-moistened.

[0034] The results indicate that the EBCT of 60 and 90 seconds workedbetter than the 300 second column. The 60 second column adsorbed 45 mgof H₂S per gram of filtering media during its first cycle, graduallydeclining to an average 32 mg of H₂S per gram of filtering media percycle and a total removal of 560 mg of H₂S per gram of filtering mediain 18 cycles. Testing on the 60 second EBCT column was stopped when theremoval was only 26 mg of H₂S per gram of filtering media for the lasttwo cycles. Average outlet H₂S concentration prior to saturation wasabout 30 ppm, with several readings as low as 0.2 ppm.

[0035] The 90 second EBCT had similar results, averaging 30 mg of H₂Sremoved per gram of filtering media, and a total removal of 326 mg ofH₂S per gram of filtering media in 11 cycles. Although this column didnot remove as much H₂S per cycle, it has achieved a lower effluentconcentration of H₂S, averaging 23 ppm.

[0036] The 300 second EBCT column did not work as well as the other twocolumns. Its adsorption performance was only 20 mg/g per cycle, and itremoved 115 mg/g in 6 cycles. Average outlet H₂S concentration was 40ppm.

[0037] Further testing was carried out to measure the effectiveness ofthe filtering media in removing H₂S at different linear velocities.Linear velocity is the speed at which the gas flows vertically throughthe filtering column. Two columns were set up to operate at 1 ft/min and3 ft/min respectively, with a common EBCT of 60 seconds. The resultsshown below are compared to the 60 second EBCT column mentioned before,which was operating at 5 ft/min.

[0038] The 3 ft/min column yielded an average removal rate of 18 mg/gper cycle. The filtering column was tested through 6 cycles and adsorbed110 mg/g in total. The 1 ft/min column averaged only 17 mg/g per cycle.The column was tested through 6 cycles, and removed 100 mg/g in total.The results show that the 1 ft/min and the 3 ft/min velocities are notas effective as the 5 ft/min test which gave an average 32 mg of H₂Sremoval per gram of filtering media per cycle, and a total lifetimeremoval capacity of 560 mg of H₂S per gram of filtering media, in 18cycles.

[0039] In view of these results, a new column, 10 feet in length wasbuilt to find the maximum linear velocity which the filtering media canhandle. This column was set up to run at 60 second EBCT with a linearvelocity of 10 ft/min, through a filtering media in a dry state. Duringthree cycles, the average adsorption performance was 32 mg of H₂S pergram of filtering media. It was observed, however, that during the firstcycle, the outlet H₂S concentration never went below 50 ppm. During thesecond and third cycles, it was much lower, averaging less than 5 ppmprior to saturation. This may be explained by the higher flow rate andtherefore by a longer time required for wetting the filtering media.However, once properly wetted, performance was found to be excellent.

[0040] In commercial and industrial applications it is recommended tocontain the filtering media in a filtering column having a window orsight glass, such that users can develop certain visual skills forevaluating at a glance, the conditions of the filtering media. Duringthe regeneration of the filtering media, it is recommended to pass thepurging air exiting one filtering column into another filtering columnto capture any hydrogen sulfide that may be released from the filteringcolumn being regenerated. During regeneration of the filtering media, asmall amount of the sulphur on the ferric sulfide may be reconverted tohydrogen sulfide gas and stripped off the filtering media by theregeneration air. The amount of hydrogen sulfide exiting the filter inthe regeneration air is less than 0.02% of the hydrogen sulfide whichwas originally adsorbed by the filtering media. For environmentalreasons, it is therefore recommended to pass the purging air exiting afiltering column being regenerated into a second filtering column beforereleasing the purging air into the atmosphere. For convenience, thissecond filtering column may be an adjacent filtering column in a bank offiltering columns or a secondary filtering column provided for thispurpose.

[0041] It will be appreciated that where the application can toleratethe injection of ambient air in the gas stream, the regeneration of thefiltering media can be effected automatically on a continuing basis. Itis also believed that the filtering media can be made new again byseparating the sulphur particles from it, by washing, sifting orotherwise.

[0042] As to additional details related to the manufacturing,installation and use of the filtering media, the same should be apparentfrom the above description, and accordingly further discussion relativeto the manner of making, using and renewing the filtering media would beconsidered redundant and is not provided.

[0043] While one embodiment of the present invention has been describedherein above, it will be appreciated by those skilled in the art thatvarious modifications, alternate compositions, alternate methods andequivalents may be employed without departing from the true spirit andscope of the invention. Therefore, the above description should not beconstrued as limiting the scope of the invention which is defined by theappended claims.

We claim:
 1. A process for removing hydrogen sulfide from a gas stream,comprising the step of: passing said gas stream through a filteringmedia consisting essentially of: calcined diatomite, and between 5% and30% by weight of ferric ions bonded to said calcined diatomite; suchthat a hydrogen sulfide removal from said gas stream during a life ofsaid filtering media, is about one half the weight of said filteringmedia.
 2. The process as claimed in claim 1 , wherein said step ofpassing said gas stream through said filtering media is effected at avelocity of 5 ft/min.
 3. The process as claimed in claim 2 wherein saidstep of passing said gas stream through said filtering media is effectedat a retention time of said gas stream through said filtering media ofabout 60 seconds.
 4. The process as claimed in claim 2 , furthercomprising the step of increasing a moisture content of said filteringmedia while maintaining said velocity.
 5. The process as claimed inclaim 1 , wherein said step of passing said gas stream through saidfiltering media further comprises the step of reducing a hydrogensulfide content in said gas stream from 30,000 ppm to non-detectablelevels.
 6. The process as claimed in claim 1 , wherein said step ofpassing said gas stream through said filtering media further comprisesthe step of passing said gas stream once through said filtering mediaand simultaneously adsorbing 45 mg of hydrogen sulfide from said gasstream for each gram of said filtering media.
 7. The process as claimedin claim 1 , further comprising the step of renewing said filteringmedia when said filtering media is saturated with sulfides by blowingambient air through said filtering media.
 8. The process as claimed inclaim 7 , further comprising the step of repeating said step of renewingsaid filtering media while maintaining an average hydrogen sulfideadsorption performance thereof of 32 mg of hydrogen sulfide per gram ofsaid filtering media.
 9. The process as claimed in claim 8 , whereinsaid step of renewing said filtering media while maintaining an averagehydrogen sulfide adsorption performance thereof of 32 mg of hydrogensulfide per gram of said filtering media, is repeated 18 times.
 10. Theprocess as claimed in claim 7 , further comprising the step of changinga colour of said filtering media from orange to black and from black toorange.
 11. The process as claimed in claim 10 , further comprising thesteps of enclosing said filtering media in a filtering column having asight glass, and visually monitoring a colour change in said filteringmedia.
 12. The process as claimed in claim 7 , further comprising thesteps of enclosing said filtering media in a filtering column, andcarrying said step of renewing said filtering media in-situ within saidfiltering column.
 13. The process as claimed in claim 1 furthercomprising the step of injecting air in said gas stream.
 14. The processas claimed in claim 1 further comprising the step of causing saidfiltering media to have a pH of about 10 and favourably influencing areaction of hydrogen sulfide in said gas stream with said ferric ions insaid filtering media.
 15. A process for removing hydrogen sulfide from agas stream, comprising the steps of: passing said gas stream through afiltering media consisting essentially of: calcined diatomite, andbetween 5% and 30% by weight of ferric ions bonded by chemisorptionbonds to said calcined diatomite, and renewing said filtering media whensaid filtering media is saturated with sulfides by blowing air throughsaid filtering media; repeating said step of renewing said filteringmedia while maintaining an average hydrogen sulfide adsorptionperformance thereof of 32 mg of hydrogen sulfide per gram of saidfiltering media.
 16. The process as claimed in claim 15 , wherein saidstep of renewing said filtering media while maintaining an averagehydrogen sulfide adsorption performance thereof of 32 mg of hydrogensulfide per gram of said filtering media is repeated several times untila lifetime hydrogen sulfide adsorption capacity of said filtering mediahas reach about half the weight of said filtering media.
 17. The processas claimed in claim 15 , wherein said step of renewing said filteringmedia while maintaining an average hydrogen sulfide adsorptionperformance thereof of 32 mg of hydrogen sulfide per gram of saidfiltering media is repeatable 18 times.
 18. A process for removinghydrogen sulfide from a gas stream, comprising the steps of: passingsaid gas stream through a filtering media consisting essentially of:calcined diatomite having particles ranging in sizes between about 30mesh and about 60 mesh, and between 5% and 30% by weight of ferric ionsbonded by chemisorption bonds to said calcined diatomite, such that saidprocess is efficient and said filtering media remains porous in use whensaid gas stream is moist.
 19. The process as claimed in claim 18 ,wherein said gas stream is moist, and said step of passing said gasstream through a filtering media comprises the step of simultaneouslyadsorbing moisture from said gas stream in said filtering media andmaintaining particle sizes in said filtering media between about 30 meshand 60 mesh.
 20. The process as claimed in claim 18 , further comprisingthe steps of regenerating said filtering media by passing ambient airthrough said filtering media, and repeating said steps of passing saidgas stream through said filtering media and regenerating said filteringmedia until a cumulative hydrogen sulfide removal capacity of saidfiltering media has reached about one half the weight of said filteringmedia, while maintaining a single-pass hydrogen sulfide removalperformance of 32 mg per gram of said filtering media.